Both Uses of

demeanor
in
A Tale of Two Cities

Something especially reckless in his demeanour, not only gave him a disreputable look, but so diminished the strong resemblance he undoubtedly bore to the prisoner (which his momentary earnestness, when they were compared together, had strengthened), that many of the lookers-on, taking note of him now, said to one another they would hardly have thought the two were so alike.†
Chpt 2.3 *

unconventional spelling: This is a British spelling. Americans use demeanor.

The smooth manner of the spy, curiously in dissonance with his ostentatiously rough dress, and probably with his usual demeanour, received such a check from the inscrutability of Carton,—who was a mystery to wiser and honester men than he,—that it faltered here and failed him.†
Chpt 3.8

Definitions:

(1) (demeanor)
the manner in which a person behaves
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Much more rarely, the verb form may be used as a synonym for behave as in: "Try to demean yourself with dignity."